Portuguese police are investigating the controversial transfer of Bébé to Manchester United amid suspicions that much of the transfer fee remains unaccounted for.

United signed the Portugal Under-21 player from Vitória de Guimarães for £7.4m in mid-2010 even though Sir Alex Ferguson later admitted he had never seen the forward play.

An anti-corruption department of the Portuguese police service was reported on Friday to be looking into the transfer, the Times said. Bébé, 21, started only three matches before joining Besiktas on a season-long loan last summer.

But he made his debut for the Turkish club only last month after he tore a cruciate ligament in his left knee while playing for the Portugal Under-21 side.

Ferguson admitted that Bébé had been an impulse buy. "It is one of those things that happens when you identify someone with potential," Ferguson said. "Normally you would assess someone over a longer period of time but other clubs were starting to have a look so we made a quick decision."